The present invention relates to an improved magnetic recording medium. Particularly, it relates to a magnetic recording medium excellent in dispersibility, thermal stability and durability.
A magnetic recording medium such as magnetic tape or magnetic card is generally prepared by applying a magnetic paint comprising a magnetic powder and a binder to a substrate such as polyester film to form a magnetic layer on the substrate.
Recently, a finely divided magnetic powder having a large specific surface area has been used in the above preparation of a magnetic recording medium with the purpose of enhancing the coercivity and the maximum amount of saturation magnetization to thereby improve the signal-to-noise ratio and the magnetic recording density.
However, when a magnetic paint is prepared by using a finely divided magnetic powder and a vinyl chloride copolymer resin binder such as vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin, vinyl chloride-vinyl acetate-maleic acid copolymer resin or vinyl chloride-vinyl acetate-maleic acid-vinyl alcohol copolymer resin, there occurs a problem of increase in the viscosity of the paint or lowering in the dispersibility of a finely divided magnetic powder in such a binder.
In order to improve the dispersibility of a finely divided magnetic powder, a high-shear dispersion process has been applied to the dispersion of the powder.
However, this high-shear dispersion process causes thermal decomposition of the vinyl chloride copolymer resin owing to both viscosity increase of the magnetic paint and temperature increase thereof to generate hydrogen chloride gas, which degrades the magnetic powder to lower the durability of the magnetic recording medium, thus disadvantageously damaging the reliability thereof.
Meanwhile, it has been well known that a stabilizer is added to a vinyl chloride copolymer resin to inhibit the thermal decomposition thereof.
Representative examples of such a stabilizer include low-molecular weight epoxy compounds such as epoxidized soyabean oil and n-butyl glycidyl ether and liquid organotin compounds such as dibutyltin laurate and dibutyltin maleate.
However, when a low-molecular weight epoxy compound is contained in a magnetic recording medium in a large amount, it tends to bleed from the medium to cause lowering in the durability of the medium and staining of a head. Therefore, the amount of a low-molecular weight epoxy compound to be added is restricted in itself, so that the effect thereof is also restricted.
On the other hand, when an organotin compound is added to the magnetic paint, this organotin compound acts as a catalyst for the crosslinking of an isocyanate compound, which is ordinarily added to the paint, to extremely shorten the pot life of the paint. Such a lowering in the stability of the magnetic paint brings about significant increase in the viscosity of the paint during its application to lower the dispersibility and surface smoothness of the magnetic layer.
Meanwhile, a process for crosslinking a magnetic coating film with electron beams has been proposed with the purpose of preparing a magnetic recording medium free from a problem of pot life and excellent in durability.
However, even this process using electron beams cannot give a desirable magnetic recording medium, because the binder and additives to be used in the process do not always exhibit satisfactory performances.